The present invention relates to a solid state image sensor and a method for fabricating the same, and more specifically to a solid state image sensor having a charge transfer electrode formed by patterning a single-layer conducting electrode material film.
In the prior art, this type of solid state image sensor includes a solid state image sensor having a photoelectric conversion section formed of a buried photodiode, as shown in FIGS. 1A to 1D, which are diagrammatic sectional views for illustrating a process for fabricating the prior art solid state image sensor.
First, after a first P-type well 32 and second P-type wells 33 are formed on an N-type semiconductor substrate 31 by means of a thermal diffusion, phosphorus is ion-implanted to form a vertical charge transfer section 34. Thereafter, boron is ion-implanted to form a channel stopper region 36 and an electric charge reading region 35, as shown in FIG. 1A.
A surface of the substrate 31 thus formed is thermally oxidized to form a gate oxide film 37, and a charge transfer electrode material film 38 is deposited on the gate oxide film 37 by means of a low pressure CVD (chemical vapor deposition). Thereafter, a patterned photo resist layer 39 is formed for formation of a reading electrode, as shown in FIG. 1B.
A dry etching is carried out by using the photo resist 39 as a masks to form a charge transfer electrode 40. Phosphorus ions (P+) are ion-implanted in self-alignment manner using the charge transfer electrode 40 covered with the photo resist 39, as a mask, so as to form an N-well 41, which becomes a photodiode, as shown in FIG. 1C. At this time, the photo resist 39 has a thickness of about 3 xcexcm enough to prevent the phosphorus ions from passing through the photo resist 39.
Thereafter, in order to form a buried photodiode, the photo resist 39 is removed, and then, boron ions (B+) are ion-implanted using the charge transfer electrode 40 as a mask, to form a P+ region 42, as shown in FIG. 1D.
FIGS. 1A to 1D show the sectional views of the device in the course of fabricating the solid state image sensor. A layout pattern of the electrodes in a plan view becomes as shown in FIG. 2. A sectional view taken along the line Bxe2x80x94B in FIG. 2 corresponds to the sectional view of FIGS. 1A to 1D. The layout pattern shown in FIG. 2 is disclosed in Japanese Patent Application Pre-examination Publication No. JP-A-08-293592, (an English abstract of JP-A-08-293592 is available from the Japanese Patent Office and the content of the English abstract of JP-A-08-293592 is incorporated by reference in its entirety into this application).
The transfer electrode is constituted of a plurality of polysilicon layers (two layers in the example shown in FIG. 2). The transfer electrode includes a first layer electric transfer electrode 52 and a second electric transfer electrode 53 in the form of a comb having each projection positioned on one vertical charge transfer section 51, the first layer electric transfer electrode 52 and the second electric transfer electrode 53 being located to face to each other in a plan view in such a manner that each projection of the first layer electric transfer electrode 52 overlaps on a tip end of a corresponding projection of the second electric transfer electrode 53.
Incidentally, the solid state image sensor shown in FIG. 2 has such an arrangement that two electrodes are located adjacent to each other for each one photo electric conversion section 54. In this construction, it is not possible to simultaneously read out a signal electric charge from all the photo electric conversion sections 54.
In order to simultaneously read out the signal electric charge from all the one photo electric conversion sections 54, it is necessary to arrange three electrodes adjacent to each other for each one photo electric conversion section 54. For example, the arrangement as shown in FIG. 3 is adopted. In the example shown in FIG. 3, the transfer electrode includes three layers of conducting electrode material film, namely, a first layer electric transfer electrode 62, a second layer electric transfer electrode 63 and a third layer electric transfer electrode 64. A sectional view taken along the line Cxe2x80x94C in FIG. 3 corresponds to the sectional view of FIGS. 1A to 1D.
In the above mentioned prior art solid state image sensor, since the charge transfer electrode is constituted of a plurality of layers of conducting electrode material film, it is necessary to form a relatively thin interlayer film in order to ensure a sufficient insulation between the plurality of layers of electrode, with the result that a height of the device become high, and therefore, it becomes difficult to work. In addition, the step coverage of a light blocking or shielding film lowers, and a smear property is deteriorated.
In order to overcome the above mentioned problem, it may be considered to constitute the charge transfer electrode by a single-layer conducting electrode material film. In this case, in order to stabilize a reading characteristic for reading a signal charge from the photoelectric conversion section to the vertical charge transfer section, it is necessary to consider the plan pattern of the electrodes to form the photoelectric conversion section in self alignment to the vertical charge transfer electrode, so that when the ion-implantation is carried out for forming the photoelectric conversion section, an ion will never be implanted into a gap region between the electrodes.
Accordingly, it is an object of the present invention to provide a solid state image sensor and a method for fabricating the same, which have overcome the above mentioned problem of the prior art.
Another object of the present invention is to provide a solid state image sensor and a method for fabricating the same, capable of preventing a positional deviation between the photoelectric conversion section and the vertical charge transfer electrode, thereby to stabilize a reading characteristic for reading a signal charge from the photoelectric conversion section to the vertical charge transfer section.
The above and other objects of the present invention are achieved in accordance with the present invention by a solid state image sensor comprising a plurality of photoelectric conversion sections formed at a surface region of a first conductivity type semiconductor layer, a charge transfer section of a second conductivity opposite to that of the first conductivity type semiconductor layer, the charge transfer section being formed in the surface region of the first conductivity type semiconductor layer, adjacent to the photoelectric conversion sections, to transfer a signal charge along the charge transfer section, a plurality of charge reading sections each formed in the surface region of the first conductivity type semiconductor layer, between a corresponding one of the photoelectric conversion sections and the charge transfer section, for reading out a signal charge generated in the corresponding photoelectric conversion section, to the charge transfer section, and tranfer electrodes formed of a single layer of conducting electrode material to cover through a gate insulator film the charge reading sections and the charge transfer section, wherein the tranfer electrodes are formed by selectively etch-removing the single layer of conducting electrode material at a plurality of first regions which divide the single layer of conducting electrode material in a row direction for each one pixel, and at a second region positioned above each of the photoelectric conversion sections, the first regions and the second region being not overlapped to each other.
According to another aspect of the present invention, there is provided a method for fabricating a solid state image sensor which comprises a plurality of photoelectric conversion sections formed at a surface region of a first conductivity type semiconductor layer, a charge transfer section of a second conductivity opposite to that of the first conductivity type semiconductor layer, the charge transfer section being formed in the surface region of the first conductivity type semiconductor layer, adjacent to the photoelectric conversion sections, to transfer a signal charge along the charge transfer section, a plurality of charge reading sections each formed in the surface region of the first conductivity type semiconductor layer, between a corresponding one of the photoelectric conversion sections and the charge transfer section, for reading out a signal charge generated in the corresponding photoelectric conversion section, to the charge transfer section, and tranfer electrodes formed of a single layer of conducting electrode material to cover through a gate insulator film the charge reading sections and the charge transfer section, the method including the steps of:
forming the single layer of conducting electrode material through the gate insulator film on a surface of the first conductivity type semiconductor layer;
etch-patterning the single layer of conducting electrode material for dividing the single layer of conducting electrode material in a row direction at a plurality of first regions;
forming a patterned mask to cover the first regions and the single layer of conducting electrode material but to expose the single layer of conducting electrode material at a second region above each of the photoelectric conversion sections;
selectively etch-removing the single layer of conducting electrode material using the patterned mask as a mask; and
removing the patterned mask.
In other words the solid state image sensor in accordance with the present invention comprises the plurality of photoelectric conversion sections formed at the surface region of the first conductivity type semiconductor layer, the charge transfer section of the second conductivity opposite to that of the first conductivity type semiconductor layer, the charge transfer section being formed in the surface region of the first conductivity type semiconductor layer, adjacent to the photoelectric conversion sections, to transfer the signal charge along the charge transfer section, the plurality of charge reading sections each formed in the surface region of the first conductivity type semiconductor layer, between the corresponding one of the photoelectric conversion sections and the charge transfer section, for reading out the signal charge generated in the corresponding photoelectric conversion section, to the charge transfer section, and tranfer electrodes formed by patterning the single layer of conducting electrode material formed to cover through the gate insulator film the charge reading sections and the charge transfer section.
In the solid state image sensor in accordance with the present invention, the tranfer electrodes are formed by selectively etch-removing the single layer of conducting electrode material at the plurality of first regions which divide the single layer of conducting electrode material in the row direction for each one pixel, and at the second region positioned above each of the photoelectric conversion sections, the first regions and the second region being not overlapped to each other.
Preferably, the photoelectric conversion sections are formed in a self alignment with the second regions where the single layer of conducting electrode material has been selectively removed from a position above each of the photoelectric conversion sections.
The tranfer electrodes are applied with charge transferring pulses which have different timings, respectively.
In addition, a gate electrode in a peripheral circuit of the solid state image sensor is formed by etch-patterning the single layer of conducting electrode material so that the gate electrode in the peripheral circuit is formed in the same etching process as the second region is formed by selectively removing the single layer of conducting electrode material from a position above each of the photoelectric conversion sections.
Furthermore, the solid state image sensor in accordance with the present invention further includes a peripheral circuit including a first transistor having a gate insulator film of the same film thickness as that of the gate insulator film in the charge transfer section, and a second transistor having a gate insulator film of a film thickness thinner than as that of the gate insulator film of the first transistor, and a gate electrode of the first and second transistors in the peripheral circuit is formed by etch-patterning the single layer of conducting electrode material.
For example, the gate electrode of the second transistor is formed at the same etching process as the single layer of conducting electrode material is etch-removed at the first regions, and the gate electrode of the first transistor is formed at the same etching process as the single layer of conducting electrode material is etch-removed the second region.
As mentioned above, the method in accordance with the present invention for fabricating the solid state image sensor comprises the steps of forming the single layer of conducting electrode material through the gate insulator film on the surface of the first conductivity type semiconductor layer, etch-patterning the single layer of conducting electrode material for dividing the single layer of conducting electrode material in the row direction at the plurality of first regions, forming the patterned mask to cover the first regions and the single layer of conducting electrode material but to expose the single layer of conducting electrode material at the second region above each of the photoelectric conversion sections, selectively etch-removing the single layer of conducting electrode material using the patterned mask as the mask, and removing the patterned mask.
With this method, it is possible to fabricate the solid state image sensor which comprises the plurality of photoelectric conversion sections formed at the surface region of the first conductivity type semiconductor layer, the charge transfer section of the second conductivity opposite to that of the first conductivity type semiconductor layer, the charge transfer section being formed in the surface region of the first conductivity type semiconductor layer, adjacent to the photoelectric conversion sections, to transfer the signal charge along the charge transfer section, the plurality of charge reading sections each formed in the surface region of the first conductivity type semiconductor layer, between the corresponding one of the photoelectric conversion sections and the charge transfer section, for reading out the signal charge generated in the corresponding photoelectric conversion section, to the charge transfer section, and tranfer electrodes formed by patterning the single layer of conducting electrode material formed to cover through the gate insulator film the charge reading sections and the charge transfer section.
In the method in accordance with the present invention for fabricating the solid state image sensor, after the single layer of conducting electrode material is selectively etch-removed using the patterned mask as the mask, a first conductivity type impurity and a second conductivity type impurity are ion-implanted using the patterned mask and the single layer of conducting electrode material, or only the single layer of conducting electrode material as a mask, to form the photoelectric conversion section.
Alternatively, after the single layer of conducting electrode material is selectively etch-removed using the patterned mask as the mask, a second conductivity type impurity is ion-implanted using the patterned mask and the single layer of conducting electrode material, or only the single layer of conducting electrode material as a mask, and a first conductivity type impurity is ion-implanted into a surface region of the second conductivity type impurity region in a self alignment, using the charge transfer electrodes as a mask.
Furthermore, the second conductivity type impurity can be ion-implanted from an inclined direction, so that the second conductivity type impurity region is formed to extend under the charge transfer electrode, but in a self alignment with but separately from a transfer electrode end of the charge reading section.
In addition, the first conductivity type impurity can be ion-implanted from an inclined direction, so that the first conductivity type impurity region is formed in a self alignment with but separately from a transfer electrode end of the charge reading section.
As seen from the above, according to the present invention the charge transfer electrodes are formed by etch-patterning the single layer of conducting electrode material, which dividing an etch-removed region into a plurality of first regions which divide the single layer of conducting electrode material into a plurality of electrodes in a row direction, and a second region positioned at the photoelectric conversion section. After the second region is etch-removed, an impurity is implanted using the remaining electrodes as a mask to form for example an N-well which becomes the photoelectric conversion section. Therefore, the photoelectric conversion section and the signal charge reading electrode can be formed with no positional deviation therebetween, so that it becomes possible to stabilize a reading characteristics for reading a signal charge from the photoelectric conversion section to the vertical charge transfer section.
Thus, in the solid state image sensor having the charge transfer electrode of the single-layer structure in accordance with the present invention, the charge transfer electrode material film is deposited on the whole surface of the device, and then patterned into the charge transfer electrodes are separated from one another by the grooved separation regions, for each one pixel. Therefore, the opening for forming the photoelectric conversion section is formed in the charge transfer electrode, of the charge transfer electrodes, which also functions as a signal charge reading electrode for reading a signal charge from the photoelectric conversion section to the vertical charge transfer section, without overlapping with the grooved separation regions. Furthermore, the ions are implanted using the photo resist used for forming the opening and the charge transfer electrodes as a mask. Thus, the photoelectric conversion section and the signal charge reading electrode can be formed with no positional deviation therebetween, so that it become possible to stabilize a reading characteristic for reading a signal charge from the photoelectric conversion section to the vertical charge transfer section.